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Black Holes' Growth Measured
Tei'ehm Teuw writes: "In an article from NASA@Today.gov, astronomers
are concluding that monstrous black holes weren't
simply born big but instead grew on a measured diet of gas and
stars controlled by their host galaxies in the early formative
years of the universe. These results, gleaned from a
NASA Hubble Space Telescope census of more
than 30 galaxies, are painting a broad picture of a galaxy's evolution and its long
and intimate relationship with its central giant black hole.
Though much more analysis remains, an initial look
at Hubble evidence
favors the idea that titanic black holes co-evolved with the galaxy by trapping a
surprisingly exact percentage of the mass of the central hub of stars and
gas in a galaxy." This seems an affront to my simplistic understanding of black hole behavior, but heck, we're not even sure black holes exist, anyhow.
Black holes do in fact lose mass to Hawking radiation, but the amount of energy/mass lost to this effect is so fantastically small that any infalling objects/energy more than makes up for it. The "evaporation" of black holes through Hawking radiation doesn't become a significant effect until the hole approaches microscopic sizes, simply because evaporation time is proportional to the mass of the hole cubed. For a black hole of roughly 30 solar masses that has no infalling matter it would take 10^61 times the age of the universe to evaporate. Black holes with mass less than 10^11 kg can evaporate in less than the age of the universe. [Ref]
Hawking radiation isn't really energy escaping the black hole as that is not allowed for by their very definition. The event horizon is defined as the point at which the curvature of space-time becomes so great that not even photons can escape its pull (not a strict definition I know, but a functional one at least).
HR is actually something which occurs on the "surface" of the black hole, at the event horizon. It involves Heisenberg's uncertainty principle, and particle/anti-particle pairs as I undertand it, but would not venture an explanation as I am sure it would be sorely inadequate.
As far as photons travelling at a velocity greater than c, I don't know what to say. I'm under the impression that our current understanding of physics indicates that superluminous velocities under any circumstance breaks a lot of what we think we know about the universe (especially special relativity and I think parts of general relativity). What effect that would have on black holes (their very existence in particular) I wouldn't want to guess.
Sorry, but that's just plain wrong. Any particle created within the event horizon must be created with sufficient kinetic energy to overcome the force of gravity and escape the potential well of the hole.
:-)
:-) that it is the particle with positive energy that escapes, and the one with negative energy that is swallowed. Thus, the net total energy of the hole is reduced, and due to energy-mass equivalnce, it's mass is reduced.
The reason that we cannot see black holes optically, is that light does not have sufficient kinetic energy to escape the gravitaitonal field at a distance outside the event horizon. What you propose here is that particles are created that have supra-light velocity; this is expressly forbidden by relativity.
You are on the right track, however
The theory is that, due to quantum effects, a matter/anti-matter particle pair can spontaneously be created near the event horizon. (This happens in ordinary space all the time, and is permitted by energy conservation, over short time scales).
If the pair is created at the right distance from the horizon, then it is possible that one particle will have sufficient energy to escape, while the other (travelling in the opposite direction) is swallowed by the hole.
It turns out (ie don't ask me to do the maths
Cheers,
Tim
It's official. Most of you are morons.
The theory is something like this:
It's been postulated that just within the 'event horizon' of a black hole (that point at which even light cannot normally escape the gravitational field), a complex particle could decay into constituent particles under the forces around it...... these constituent particles will then zoom away from each other due to the energy released and it's possible for one or many of these particles to leave the black hole before its gravity has slowed the particles sufficiently to suck them back down.
It's kinda like here on Earth - I can throw a ball up in the air and it'll fall down again and if I throw it nearly fast enough to leave the gravitational pull, it'll get hearly all the way our and then fall back again... so if I make a special ball that's designed to break in two at or near the top of it's flight and fire the two sections in opposite directions (one up and one down), then the top section now has enough velocity to escape.....
This is partially how/why multiple stage rockets are better then single stage ones in certain circumstances.....
troc
Troc's dubious podcast and blog: http://www.trocnet.net
IIRC, Hawking radiation is kept around as an explanation as to why we're not bombarded with microscopic black holes formed during the big bang, more than as an explanation of how a typical run-of-the-mill black hole should shrink over time. When you think about it, you'd have to win an unreasonable number of coin tosses for the antiparticle to get swallowed enough times for a regular black hole to evaporate completely.
"If one is really a superior person, the fact is likely to leak out without too much assistance" -- John Andrew Holmes
My apologies to Stephen Hawking for stealing the title (as best I remember it) of a chapter of "A Brief History of Time".
;-)
:-)
/. who appreciate the occasional change of pace from computer topics.
IAAPS (I Am A Physics Student, though somewhat rusty at the moment
Black holes can shrink (this is not to say they must shrink). While it may seem impossible, since matter and energy can not escape the event horizon of a black hole, black holes do radiate energy. Some other law of physics (pertaining to black body radiation, IIRC) requires that black holes radiate a certain minimum amount of energy, causing Hawking (and others) to ponder how this could be. Quantum mechanics provides a solution.
Space, even a perfect vacuum, is not devoid of matter. Although a perfect vacuum has an average energy of zero, this is only an average. Quantum mechanics allows for the spontaneous creation of "virtual" particle-antiparticle pairs, which quickly annihilate each other (virtual because they are annihilated before they ever interact with other matter). I forget the details, but the more energy such a pair has, the shorter the amount of time it can exist for--there's a Planck constant in there somewhere
The trick is that near a black hole, sometimes these particle pairs sap energy from the black hole, and at least one of the particles becomes "real". You could imagine the other particle falls into the hole or something...
As for the article about Yilmaz's version of General Relativity (which predicts the non-existance of black holes), I don't yet know enough to criticize his General Relativity on the basis of the mathematics or theoretical physics. AFAIK, most of the "observations" of black holes have simply been of the motion of stars perturbed by massive, relatively dark objects, or of radiation thrown from the disk of material spiraling into a massive black hole candidate. I don't see why a small, massive (non-black hole) object as predicted by Yilmaz couldn't have been responsible for all these effects.
Do I think Yilmaz is right? Probably not, but it would be pretty damn cool if he is. I admire the guts of physicists who do "monkey wrench physics", and dare to challenge the established theories.
Please don't flame me if I've blown some of the details; I haven't done a physics course for over a year (I've been working on the comp sci half of my degree). I would appreciate any corrections or additions, though. I hope there are lots of other physics geeks on
Great... I can just picture a black hole support group.
Black Hole 1: Hi, I'm Globulax, Swallower of Galaxies... I've been a black hole for billions of years.
Rest of Black Holes (seated on uncomfortable folding chairs): Hi, Globulax!
Black Hole 2: It's okay... we're all Black Holes here... nobody's judging you...
Globulax: <sniffle> Well, I wasn't born big... I was raised on a measured diet of gas and stars controlled by my host galaxy...
Black Hole 2: Don't worry... with time, you'll learn to control your cravings...go on... it's good to let these emotions out...
Globulax: I feel so unwanted... so unloved... even light tries to escape my event horizon... <sob>... and... and... scientists aren't sure if I even exist! Waaaaaaaaaaaaahhhhhhh!!!
Black Hole 2: There, there. It's okay, Globulax. Come on, everybody, let's all give Globulax a big group hug!
The Black Holes sympathetically converge near Globulax. Unfortunately, their gravity is too strong, and they swallow each other into one ultra-massive Super Hole, whose gravity gobbles up everything in the vicinity and rips a large tract of spacetime to shreds...
Black Holes (in unison): Oops!
Black Hole 2: Great... there go our folding chairs...
Stay up hacking each weekend. Sleep is for the week.
There are no "300x the speed of light" microwaves. This merely an effect that 'appears' to travel faster than light. Note that the article in that story specified that the effect only occurs with certain very specifically shaped waveforms. This takes the mystery out of how the far end of the apparatus can 'reconstruct the complete waveform' when only the leading edge has had time to enter the apparatus.
/.ers hold dear)
.999 the speed of light
Such 'apparently faster than light' effects are not uncommon in relativistic phenomena. One example is the 'superluminal fireball' from the 80's:
Imagine a star 1000 light years from Earth gave off a 'fireball' or giant plasma burst pointed at our planet in the year 1000 AD. For the sake of argument let's say it's travelling at 99.9% the speed of light. In the year 2000, the light, we would see the light from the explosion, and could 'watch the fireball eject'. Meanwhile, the fireball itself will travel 999 light years, and be just 1 light year from Earth. Therefore the light that the fireball gives off in 2000 will reach the Earth in 2001 (followed, four days later, by the fireball itself striking the earth, possibly doing nasty things to humanity, Linux, Natalie Portman, and other things
To an observer on Earth, the fireball will appear to have travelled 999x the speed of light. To an observer somewhere at right angles to the fireball's path, it will appear to travel at
If you can go to bed, knowing you did a valuable thing today, you're very lucky. If you can't... it's not bedtime
I'll concede the first point, as I'm not entirely sure, and have no Physics texts to hand (as I'm at work :-) )
:-)
(Although, now I think of it, the photon has momentum, and so (as the two are related), probably does have KE; I'm willing to be proved wrong, however)
The second point, however, I feel I must contest. An infinite red-shift is identical to an infinite increase in wavelength. This is, in turn, identical to an infinite reduction in energy (the longer the wavelength of the photon, the lower its energy - E = h/lambda IIRC).
As the photon has no rest mass, it is pure energy, and so with its energy reduced to zero, it has effectively "destroyed" itself in its efforts to escape the event horizon.
No energy means no photon; no photon means nothing escaped
(And before anyone starts shouting, no, you can't destroy energy; I'm guessing that the energy of the photon has gone to reduce the potential energy of the gravitational field)
Cheers,
Tim
It's official. Most of you are morons.
A black hole suspended in an absolute vacuum (no matter, no radiation, nothing) will slowly radiate at a temperature which is inversely proportional to its surface area, and thus shrink.
Now let's say we have a black hole and an external background radiation. Black hole now absorbs radiation and grows (but still continue to shrink at the same time). If the temperature of the radiation is greater than that of black hole, the latter would grow faster than it shrinks. Now throw in some matter, and the black hole will grow even faster.
BTW microwaves don't travel 300x the speed of light. Phase velocity can be anything you like, but phase does not "travel".
--
Industrial space for lease in Flatlandia.
As is, black holes have a definite, but not necessarily measurable mass... it's just the mass they started with plus the mass of whatever they have eaten and minus what has been lost to Hawking radiation.
Now, the singularity... yes it does supposedly approach infinite density (approach is an important mathematical distinction), and is even capable of shredding the very Einsteinian laws that define it. The only reason it's tolerated, physicists say (kind of a strange reason, but I don't have the tools to judge) is because the singularity is locked away, and no information can be transmitted about it. The universe doesn't know that there's this strange point at where space-time curvature (gravity) approaches infinity.
To make things really weird, Brian Greene added a nifty component to string theory (which is what gave him credibility for his book besides being a damned good physicist). In the 6 curled dimensions, there is a transformation was allows space to remain continuous, but plants the seed for a black hole... That folding may be part of what keeps the universe from tearing itself apart during black hole formation.
Either way, its pretty nifty how the 'weakest' force in the universe can kick the other three's asses when enough mass is involved....